Use the loki_ prefix for built files

[janvdijk]

Some tests are called test_xxx, instead of loki_test_xxx. We should use the loki_ prefix consistently to avoid name clashes when the project is built in a subdirectory of another cmake project (names such as test_grid are too common).

Or should we use lokib_ for all binaries and other outputs of this project? To be discussed. @daanboer, @wgraef, what do you think?

[wgraef]

I agree with using a prefix. For future-proofing, I'd say lokib_ to avoid possible confusion with loki-c or loki-mc (though I don't know how realistic that is).